Method and device for controlling an alternator for a motor vehicle

ABSTRACT

Method and device ( 1 ) for controlling an alternator ( 2 ) delivering an electrical current which supplies power to electrical consumers ( 4 ) in a motor vehicle equipped with an internal combustion engine and which recharges a battery ( 3 ), in which method and device, for each journey, the intensity of an excitation current (Ie) for the alternator is controlled as a function of a time profile Ie(t) constructed from a probable time profile (Ωa (t)) of the rotary speeds of the alternator and from the current (Ic) absorbed by the consumers. Preferably, the time profile of the rotary speeds of the alternator is determined from data provided by a navigation device ( 7 ) optionally combined with a traffic density information receiver ( 7′ ).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and a device for controllingan alternator for a motor vehicle, of the type in which the intensity ofan excitation current for the alternator is controlled depending on theoperating conditions of the engine and of the vehicle.

2. Description of the Related Art

Known from the prior art, especially from document FR 2,594,273, is adevice for controlling an alternator which allows its excitation currentto be modulated depending on the operating conditions of the engineand/or of the vehicle so as to overexcite it when decelerating and tode-excite it when accelerating. Such a device makes it possible toimprove the overall efficiency of the engine/alternator assembly.However, this improvement is limited by the need to restrain thede-excitation phases so as not to discharge the battery excessively.This is because such a device, looking only at the instantaneousoperating conditions of the engine, may not allow a de-excitation phaseto be maintained below a certain level of charge of the battery andforces the alternator to be put back into service even if conditionsmore propitious to recharging the battery should arise subsequently.

SUMMARY OF THE INVENTION

The object of the present invention is therefore to provide a method anda device for controlling an alternator for a motor vehicle which makesit possible to take into account the operating conditions over theentire journey and to improve the optimization of the alternator/engineassembly.

These objects of the invention are achieved, as well as others whichwill appear in the rest of the present description, by means of a methodfor controlling an alternator delivering an electrical current whichsupplies power to electrical consumers in a motor vehicle equipped withan internal combustion engine and which recharges a battery, in whichmethod the intensity of an excitation current for the alternator iscontrolled depending on operating conditions of the engine and of thevehicle over a journey. According to the invention, for each journey, aninitial state of charge of the battery is determined, a probable timeprofile of the rotary speeds of the alternator is computed for thisjourney, the current absorbed by the consumers is measured, an optimumtime profile of the excitation current for the alternator is determinedsuch that, at the end of the journey, the final state of charge of thebattery is at least equal to a predetermined threshold, and theintensity of the excitation current is controlled according to thecurrent profile determined. Advantageously, the measurement of thecurrent absorbed is periodically updated and the optimum excitationprofile to be applied to the alternator is recomputed at each update.

According to a first embodiment of the invention, the time profile ofthe rotary speeds of the alternator is computed from recordings madeduring previous journeys.

According to a second embodiment of the invention, the time profile ofthe rotary speeds of the alternator is computed from informationrelating to the intended journey and provided by a navigation device onboard the vehicle.

Advantageously, in this embodiment, information relating to the densityof the traffic over the journey in question, supplied by an RDS-typetraffic information receiver coupled to the navigation device, is alsoused.

According to an important characteristic of the invention, the optimumtime profile of the excitation current is computed in such a way thatthe efficiency of the alternator is a maximum if its rotary speed isgreater than a predetermined threshold and the excitation current iszero otherwise.

The invention also provides a device for implementing the method,comprising, in combination, means of communication with at least one ofthe engine management and navigation devices, measurement means suitablefor determining the state of charge of the battery and the intensity ofthe current delivered to the electrical consumers in the vehicle,computing means and storage means suitable for computing, frominformation transmitted by the communication means, at the start of eachjourney, a probable time profile of the rotary speeds of the alternatorfor this journey and for determining, depending on the informationprovided by the measurement means, an optimum time profile of theexcitation current for the alternator such that, at the end of thejourney, the final state of charge of the battery is at least equal to apredetermined threshold, and means for controlling the intensity of theexcitation current delivered to the alternator.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will appear on readingthe description which follows and on examining the appended drawings inwhich:

FIG. 1 shows a functional diagram of the device according to theinvention;

FIGS. 2A and 2B show comparative time graphs of the state of charge ofthe battery as a function of a time profile for the rotation of thealternator, these being obtained using a method of the prior art andusing the method according to the invention, respectively, and

FIG. 3 shows a diagram of the efficiency curves of an alternator, thesebeing stored in the device and used by the method.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a computer 1 comprising means 15 for controlling anexcitation current Ie for an alternator 2 of a motor vehicle. Thealternator 2 is rotated by the engine of the vehicle (not shown) anddelivers an electrical current Ia=Ib+Ic to a battery 3 and to electricalconsumers 4. Conventionally, depending on the excitation current Ie, onthe rotary speed Wa of the alternator and on the current Ic supplied tothe consumers 4, the current output by the alternator may be sufficientto provide a current Ib for recharging the battery, or may be topped upby the current Ib, then of negative sign, coming from the battery. Thesecurrents are measured by suitable sensors, for example Hall-effectsensors 8 and 9, the signal from which is transmitted to measurementmeans 14 included in the computer 1.

Advantageously, when the computer 1 is also used as a central processingunit for controlling the consumers in the vehicle, the current Ic may bededuced from the state of control of these consumers. Likewise, thevoltage Vb across the terminals of the battery may be transmitted viathe sensor 8 to the measurement means 14, or measured directly by notingthat the computer 1 itself is a consumer supplied by the voltage Vb. Thecomputer 1 furthermore includes computing means 11, such as amicroprocessor, and storage means 12, such as ROM and RAM memoriessuitable for containing a program executable by the microprocessor anddata which will be explained later. The computer 1 is also provided withcommunication means, such as an interface for a VAN- or CAN-typecommunication network, allowing it to receive information coming fromother pieces of equipment in the vehicle such as, in this case, therotary speed N of the engine provided by an ignition or injectioncomputer 5, information relating to the data and time, supplied by adevice for informing the driver 6 and itinerary information provided byan onboard navigation computer 7. The navigation computer 7 mayadvantageously be coupled to a traffic information receiver 7′ of theRDS type, making it possible to supplement the itinerary informationwith the traffic density over this itinerary.

The operation of the device in FIG. 1 will now be described in relationto the graphs in FIGS. 2A and 2B, these showing the operation of thedevices of the prior art and of that of the invention, respectively.Shown by dotted lines in these graphs is a time profile Ωa(t) of therotary speed of the alternator such that, from t₀ to t₂, this speed isequal to a value Ω_(idle) corresponding approximately to the speed ofthe alternator when the engine is idling and, beyond t₂, this speed isvery much greater, for example corresponding to an engine speed of about4500 revolutions per minute. Shown as a solid line is the variation of aSOC parameter representative of the state of charge of the battery 3.This parameter may be simply estimated as a function of the voltage Vbacross the terminals of the battery or, better still, computediteratively as a function of the voltage Vb, the current Ib and time. Itmay be seen in FIGS. 2A and 2 B that the alternator has been de-excitedin order to relieve the engine while it is idling (rotary speed of thealternator=Ω_(idle)). Consequently, the state of charge SOC of thebattery 3 decreases uniformly, the latter supplying only the consumers4. When, at the time t1, this state of charge reaches a minimumthreshold SOC_(min), the devices of the prior art (FIG. 2A) bring thealternator back into service so as to preserve a sufficient charge atthe battery. This has the effect of applying an additional load to theengine, which may disturb its operation and cause pollution. Incontrast, in FIG. 2B, according to the invention and as will beexplained later, the computer 1 has knowledge of the time profile Ωa(t)of the rotary speeds of the alternator and knows that, after the timet₂, lying in the future, rotary speed conditions of the alternatorallowing efficient recharging of the battery will be encountered. Thecomputer can then determine, depending on the current Ic absorbed by theconsumers in the vehicle, a time profile of the excitation current Ie(t)to be applied to the alternator after t₂, suitable for recharging thebattery at least up to the minimum threshold reached at the time t₃.Consequently the device according to the invention allows the state ofcharge to fall below the SOC_(min) threshold from t₁ to t₂ and makes itpossible to optimize the operation of the engine/alternator pair.

A first procedure for implementing the method, allowing the computer 1to provide the time profile of the rotary speeds of the alternator, willnow be given in detail. In this procedure, the computer 1 makes use onlyof information provided by the ignition or injection computer 5 and thedriver information device 6. From all this information, the computer 1constructs and stores an experimental database in the following manner:the start of a journey is detected by the computer being switched on,corresponding to the vehicle being contacted. The computer then readsthe date and time which are provided by the information device 6 andclassifies this journey in a predetermined category. Noting that mostshort journeys, over which the desired optimization is most profitable,correspond to repeated journeys, it is possible to determine a certainnumber of categories by date and time of their start, for example ajourney in the morning, on a week day if the journey starts between 6a.m. and 10 a.m., from Monday to Friday, and so on. The computer thenperiodically reads the engine speed information supplied to it by theignition computer 5 via the communication means 13 and computes from itthe rotary speed Ωa of the alternator. This speed is then recorded inthe storage means 12, in time slices corresponding to the time elapsedsince the beginning of the journey. It is possible, for example, to takethe average of the rotary speed data thus acquired with those obtainedduring previous journeys for the same journey category in the same timeslice. A probable time profile of the rotary speeds of the alternatorΩa(t) in this journey category is thus obtained. This profile will beused to determine the excitation current Ie for the alternator, as willbe seen below.

According to a second procedure for carrying out the invention, use isadvantageously made of the fact that the vehicle may be provided with anonboard navigation computer 7 in order to obtain the probable timeprofile of the rotary speeds of the alternator Ωa(t). This computer, infact, computes a predicted itinerary for the journey and providesinformation relating to each segment of the itinerary, such as thelength of the segment and the nature of the roads followed (motorway,fast lane, street, lane, etc.). Then, using a preestablished table ofrotary speeds Ωa as a function of the nature of the road, the desiredprofile may be computed. In addition, when the navigation computer iscombined with a traffic information receiver 7′, each segment of theitinerary may be assigned information relating to the density of thetraffic in this segment. It will then be possible to deduce therefromthe probable duration and driving speed over each segment and, using asimple ratio, the probable time profile of the rotary speeds of thealternator Ωa(t).

We now come to the determination of the control of the excitationcurrent Ie for the alternator. At the start of the journey (time t₀),that is to say on switching on the computer 1, the latter determines aninitial state of charge SOC0 of the battery 3 by measuring, for example,its open-circuit voltage at rest. This initial state and the measurementof the battery current Ib allow the variation in the state of charge ofthe battery 3 to be monitored throughout the journey by means of theequation: $\begin{matrix}{{SOC}_{(t)} = {{SOC}_{0} + {\int_{t0}^{t}{\alpha \cdot {I_{b}(\tau)} \cdot \quad {\tau}}}}} & (1)\end{matrix}$

in which a is a charge rating such that α=1 if I_(b) is negative (thebattery delivers current to the consumers 4) and α≦1 if Ib is positive(the battery is being recharged by the alternator).

The computer 1 then computes the time profile of the rotary speeds Ωa(t)of the alternator 2 using either of the methods given above, or else bya combination of both of them.

Next, it measures the current Ic absorbed by the consumers 4 by means ofthe sensor 9 and of the measurement means 14. Assuming that the currentIc is constant throughout the duration of the journey, it thendetermines a time profile Ie(t) of the excitation current Ie using apredetermined optimization strategy. For example, Ie may be chosen suchthat Ie is zero if Ωa is less than a threshold Ω_(idle), and Ie providesthe maximum efficiency for the alternator if Ωa is greater than thisthreshold. As may be seen in FIG. 3, which shows the efficiency curvesof the alternator which are stored in the computer 1, for a rotary speedΩ_(k), following the arrow labeled (a), the current Ia_(k) is determinedsuch that this efficiency is a maximum (in this case, 60%) and thecorresponding excitation current Ie_(k) is drawn from it. Deduced fromthis is the battery current Ib=Ia−Ic at any instant, which is applied inEquation (1) to determine the final state of charge SOC_(f) of thebattery at the end of the journey. If the final state of charge thusobtained is below a predetermined minimum value, for example 70% of therated charge of the battery, a time profile Ie(t) of the excitationcurrent is recomputed such that the current Ia delivered by thealternator is greater than the current computed previously, whilemaximizing the efficiency (arrow (b) in the figure, 50% efficiency,I′a_(k), I′e_(k)). The time profile Ie(t) thus computed is then storedin the memory 12 and applied to the alternator by the control means 15.

The profile Ie(t) is periodically updated, for example every 5 minutes,by remeasuring the current Ic absorbed by the consumers 4 andrecomputing the final state of charge of the battery.

Of course, the invention is not limited to the implementation proceduredescribed and illustrated, this having been given merely by way ofexample. By analogy with the first procedure for determining the timeprofile Ωa(t) of the speed of the alternator, it would be possible todetermine a time profile Ic(t) of the current absorbed by the consumersfrom recordings made during the previous journeys and this profile couldbe used to determine an excitation current profile by means of anoptimization method other than the “trial and error” method describedabove, while seeking, for example, to minimize an additional criterionsuch as the fuel consumption of the vehicle or the emission ofpollutants.

What is claimed is:
 1. A method for powering electrical consumers in amotor vehicle with an internal combustion engine and recharging abattery, which comprises: providing an alternator having a rotary speed,the alternator producing an excitation current; controlling an intensityof an excitation current for the alternator in dependence on anoperating condition of the engine and of the vehicle during each trip,by a) determining an initial state of charge of the battery; b)computing a probable time profile of the rotary speeds of the alternatorfor the respective trip; c) measuring a current absorbed by theconsumers; d) determining an optimum time profile of the excitationcurrent for the alternator such that a final state of charge of thebattery at the end of the trip is at least equal to a predeterminedthreshold; and e) controlling the intensity of the excitation current inaccordance with the optimum time profile.
 2. The method according toclaim 1, which comprises periodically repeating steps c) and d)throughout a duration of the trip.
 3. The method according to claim 1,wherein the computing step comprises calculating the time profile of therotary speeds of the alternator from recordings made during previoustrips.
 4. The method according to claim 1, which comprises computing thetime profile of the rotary speeds of the alternator from informationrelating to the intended trip provided by a navigation device on boardthe vehicle.
 5. The method according to claim 4, which comprisesadjusting the time profile for a density of traffic over the respectivetrip.
 6. The method according to claim 1, wherein step d) comprisescomputing the optimum time profile of the excitation current such thatan efficiency of the alternator is at a maximum if the rotary speed isgreater than a predetermined threshold speed, and the excitation currentis zero if the rotary speed is not greater than the predeterminedthreshold speed.
 7. A device for controlling an excitation current foran alternator in a motor vehicle with an internal combustion engine independence on operating conditions of the engine and of the vehicleduring each trip, comprising: a communication system connected to atleast one of an engine management device and a navigation device of themotor vehicle; a measurement system for determining a state of charge ofa battery and an intensity of a current delivered to electricalconsumers in the vehicle; a computer device and a memory, said computerdevice being connected to said communication system and being programmedto: compute, from information transmitted by the communication system,at a start of each trip, a probable time profile of rotary speeds of thealternator for the trip; and determine, in dependence on informationprovided by said measurement system, an optimum time profile of anexcitation current for the alternator such that, at the end of the trip,a final state of charge of the battery is at least equal to apredetermined threshold; and a controller connected to the alternatorfor controlling the intensity of the excitation current delivered to thealternator.